1. Field of the Invention
The present invention relates to an ignition timing control system for industrial internal combustion engines, particularly for small internal combustion engines used on small boats, small generators, lawn mowers, etc.
2. Description of the Related Art
For small-sized, simple structure and low cost, industrial engines such as those used in lawn mowers, etc. have heretofore been constructed so as to be ignited simply at a previously set constant timing without making such a complicated combustion control according to a driving condition as in automotive engines while being operated steadily at a predetermined rated speed by means of a governor which mechanically regulates a throttle opening according to the engine speed.
For supply of fuel, a carburetor constructed so as to utilizes vacuum produced at a venturi of an intake passage is used so that the engine can start smoothly without supply of electric power. For cooling of the engines, air cooling which features simple structure is employed.
However, air cooling is lower in the cooling ability as compared with water cooling which is widely used for automotive engines, etc., so the carburetor for the industrial engine is set so as to allow the air/fuel mixture to be considerably richer than the stoichiometric value for thereby supplementing the cooling ability by utilizing the heat of vaporization of the mixture. For this reason, the exhaust gasses emitted from the industrial engines contain a large amount of carbon monoxide (CO), hydrocarbon (HC), etc.
On the other hand, recently, there is a tendency to require even such industrial engines to decrease in the amount of noxious components contained in the exhaust gases. To meet such a requirement, it is necessary for the air/fuel mixture to be leaner and set so as to have such an air/fuel ratio adjacent the stoichiometric value.
However, when the setting of the carburetor is caanged for simply making leaner the air/fuel mixture supplied to the engine, there is caused such a problem that not only the durability of the engine is deteriorated due to the higher combustion temperature and exhaust gas temperature but a variation of the engine speed in response to a sudden variation of the load of the engine becomes larger and therefore it takes a longer time for the engine to return to a rated engine speed, and in the worst case hunting in operation of the engine occurs.
That is, in the event the air/fuel mixture is made leaner, increase of fuel supplied to the engine when, for example, engine speed becomes lower is retarded even if the governor is operated to make larger the throttle opening since the mixture is lean. As a result, the air/fuel mixture becomes further leaner and therefore the engine speed becomes further lower. Then, the increase of fuel supplied to the engine is executed as required to increase the engine speed. However, upon this time, the throttle opening has become too large, thus causing the engine speed to become too high. Then, the governor is operated so as to decrease the throttle opening. However, upon this time, the air/fuel mixture is lean and therefore decrease of fuel is carried out rapidly, thus causing the engine speed to become lower than required. From this time onward, the above described actions are repeated to cause hunting.